The subject invention generally pertains to Telemetry, Tracking and Control (TT&C) of satellites and particularly of satellites employed in global mobile communication systems employing cellular technology.
Present spacecraft or satellites for satellite constellation systems each employ a TT&C transponder that is separate from the user voice/data communication system for such satellites. These TT&C transponders generally provide for "control" commands to be sent to the spacecraft from a fixed ground station. "Telemetry" and "tracking" information is also communicated from the spacecraft to the ground station over the TT&C transponder. Thus, such communication requires a two-way transponder link between each satellite and the ground station.
Telemetry data coming from the satellite informs a network operator about the health and status of the satellite. For example, telemetry data may include the amount of remaining hydrazine fuel for propelling rockets so that the useful life of the satellite may be estimated. Moreover, critical voltage and current magnitudes are monitored and provided as telemetry data which enables the operator to determine whether or not the circuitry of the satellite is operating properly. Tracking information includes ephemeris data which allows the location of the satellite to be determined. More specifically, a present satellite system utilizes the TT&C transponder on board the satellite to send a tone down to the base station to provide the range and the range rate of the satellite. The altitude and angle of orbit of the satellite can be computed from this information by the ground station operator. The tone may be modulated to provide a higher degree of accuracy in determining the range and range rate. The ground station provides "Control" commands in response to the tracking or telemetry data to the satellite which may be utilized to adjust the orbit of the satellite by energizing a selected jet of the satellite, for instance. Moreover, other independent control commands can be provided to reprogram the operation of the satellite to control other functions of the satellite.
The TT&C information is generally encrypted to avoid undesirable interference from the signals of other operators. Prior art systems generally only allow exchange of TT&C information with a satellite when the satellite is in line of sight with the fixed ground station. Also, prior art TT&C links are between a particular fixed ground station and its satellite and generally provide no TT&C communication link with other satellites, for instance.
TT&C transponder links, that are separate from the user voice/data channels, are presently employed on hundreds of satellites. Separate transponders are generally used because the information handled by them is generally of a different nature from the information in the user communication channels. More specifically, TT&C information may be of a predominantly digital form whereas the voice/data communication of some prior art satellite systems is of an analog form which requires all of the available bandwidth for the voice/data user communication channel. Moreover, the data rate for TT&C signals is generally much lower than for user data.
Unfortunately, utilization of the foregoing systems having separate transponders for TT&C data transfer results in several problems. Such prior art systems are not capable of mobile TT&C operation. Even in satellite constellations where user voice/data channels are interlinked between various satellites, generally the non-interlinking of TT&C transponders prevents such mobile TT&C operation. Mobile TT&C operations are advantageous for trouble shooting or for situations when the system operator is required to be at any one of various locations. Also, each satellite has only one TT&C transponder which tends to be expensive because it is vital that such transponder reliably enables the associated ground station to retain control over the satellite. Moreover, these transponders utilize electrical power obtained from the onboard power generating system which usually employs solar cells and batteries. Moreover, the use of separate TT&C transponders undesirably increases the weight of the prior art satellite systems and adds to the expense of the manufacture, testing and delivery of such satellites into orbit.